Preparation of a Diphosphine with Persistent Phosphinyl Radical Character in Solution: Characterization, Reactivity with O2, S8, Se, Te, and P4, and Electronic Structure Calculations

A new, easily synthesized diphosphine based on a heterocyclic 1,3,2-diazaphospholidine framework has been prepared. Due to the large, sterically encumbering Dipp groups (Dipp = 2,6-diisopropylphenyl) on the heterocyclic ring, the diphosphine undergoes homolytic cleavage of the P–P bond in solution to form two phosphinyl radicals. The diphosphine has been reacted with O2, S8, Se, Te, and P4, giving products that involve insertion of elements between the P–P bond to yield the related phosphinic acid anhydride, sulfide/disulfide, selenide, telluride, and a butterfly-type perphospha-bicyclobutadiene structure with a trans,trans-geometry. All molecules have been characterized by multinuclear NMR spectroscopy, elemental analysis, and single-crystal X-ray crystallography. Variable-temperature EPR spectroscopy was utilized to study the nature of the phosphinyl radical in solution. Electronic structure calculations were performed on a number of systems from the parent diphosphine [H2P]2 to amino-substituted [(H2N)2P]2 and cyclic amino-substituted [(H2C)2(NH)2P]2; then, bulky substituents (Ph or Dipp) were attached to the cyclic amino systems. Calculations on the isolated diphosphine at the B3LYP/6-31+G* level show that the homolytic cleavage of the P–P bond to form two phosphinyl radicals is favored over the diphosphine by ∼11 kJ/mol. Furthermore, there is a significant amount of relaxation energy stored in the ligands (52.3 kJ/mol), providing a major driving force behind the homolytic cleavage of the central P–P bond.